1. Field of the Invention
The present invention relates to an electronic endoscope for observing or operating on the inside of an organ, such as a stomach. Particularly, the present invention relates to adjusting the brightness of a subject image displayed on a monitor.
2. Description of the Related Art
In an electronic endoscope, light emitted from a light source passes through a fiber-optic bundle provided in a video-scope, and is radiated from the tip of the video-scope via an illuminating lens. A subject to be observed is illuminated by the radiated light, and then a subject image is formed on an image sensor via an objective lens provided at the tip portion. Image signals generated in the image sensor are read and various processes are carried out on the image signals so that the subject image is displayed on the monitor.
In general, to maintain proper brightness of the subject image, the quantity of light illuminating the subject is automatically adjusted. In a conventional electronic endoscope, for adjustment of the quantity of light, a stop (diaphragm) is provided between the light source and an incident surface of the fiber-optic bundle, and is controlled such that the brightness of the displayed subject image is maintained at a constant level. A representative luminance value, indicating the brightness of the subject image is successively calculated on the basis of the image signals. Then, the control of the stop is performed at regular time-intervals, in accordance with the difference between the calculated luminance value and a reference value indicating a proper brightness. For calculating the luminance value, namely, for the metering, the average metering or the peak metering is performed. In the case of the average metering, an average value of one frame worth of the subject image is calculated. On the other hand, in the case of the peak metering, a relatively high luminance value among the one frame worth of the subject image is defined as the representative luminance value. The operator selects the metering method as required.
The characteristics of the tip portion of the video-scope, such as the radius, the relative position of the objective lens and the illuminating lens, and so on, vary with the observed organ, such as a stomach, bronchi, a colon, and so on. Especially, the arrangement relationship between the objective lens and the illuminating lens is different. Consequently, the distribution characteristics of the light-amount on the image sensor vary with the characteristics of the tip portion. However, when the light adjustment is performed without regard to the tip characteristics, the luminance value cannot be properly calculated for a specific video-scope, so that the brightness of the subject image cannot be maintained at a proper level.